The electrical properties of polycrystalline silicon emitter (poly-emitter) bipolar junction transistors (BJT) are strongly influenced by the properties of the interfacial oxide (IFO) that exists between the polycrystalline silicon (polysilicon) emitter and the crystalline emitter. For example, the current gain (HFE) of the BJT can be enhanced by a thicker IFO layer. The current gain HFE is defined as the ratio of the corresponding change in transistor collector current δIC to a change in the transistor base current δIB. In addition to a higher HFE, an accompanying increase in the Early Voltage (Va) is obtained in PNP transistors for thicker IFO layers. The increase in HFE and accompanying increase in Va are very desirable properties of PNP transistors.
Increasing IFO thickness in NPN transistors was found to dramatically increase the low frequency output noise of the transistor (i.e., 1/f noise). This 1/f noise increase was obtained when the noise was measured using a low resistance external resistance on the base of the NPN transistor. High 1/f noise can have a detrimental effect on the use of NPN transistors in integrated circuit applications such as in the input stage of high performance operational amplifiers. Increasing the thickness of the IFO layer does not appreciably increase the 1/f noise of the PNP transistors.
There is therefore a need for a method to reduce the 1/f noise in NPN transistors containing IFO layers while maintaining the benefits obtained from such layers in PNP transistors.